If signatures are always separately retrieved and verified when
   retrieving the information they authenticate, there will be more
   trips to the server and performance will suffer.  To avoid this,
   security aware servers mitigate that degradation by always attempting
   to send the signature(s) needed.

2.3.1 The SIG Resource Record

   The syntax of a SIG resource record (signature) is described in
   Section 4.  It includes the type of the RR(s) being signed, the name
   of the signer, the time at which the signature was created, the time
   it expires (when it is no longer to be believed), its original time
   to live (which may be longer than its current time to live but cannot
   be shorter), the cryptographic algorithm in use, and the actual
   signature.

   Every name in a secured zone will have associated with it at least
   one SIG resource record for each resource type under that name except
   for glue RRs and delgation point NS RRs.  A security aware server
   supporting the performance enhanced version of the DNS protocol
   security extensions will attempt to return, with RRs retrieved, the
   corresponding SIGs.  If a server does not support the protocol, the
   resolver must retrieve all the SIG records for a name and select the
   one or ones that sign the resource record(s) that resolver is
   interested in.












 
RFC 2065                DNS Security Extensions             January 1997


2.3.2 Authenticating Name and Type Non-existence

   The above security mechanism provides only a way to sign existing RRs
   in a zone.  "Data origin" authentication is not obviously provided
   for the non-existence of a domain name in a zone or the non-existence
   of a type for an existing name.  This gap is filled by the NXT RR
   which authenticatably asserts a range of non-existent names in a zone
   and the non-existence of types for the name just before that range.

   Section 5 below covers the NXT RR.

2.3.3 Special Considerations With Time-to-Live

   A digital signature will fail to verify if any change has occurred to
   the data between the time it was originally signed and the time the
   signature is verified.  This conflicts with our desire to have the
   time-to-live field tick down when resource records are cached.

   This could be avoided by leaving the time-to-live out of the digital
   signature, but that would allow unscrupulous servers to set
   arbitrarily long time to live values undetected.  Instead, we include
   the "original" time-to-live in the signature and communicate that
   data in addition to the current time-to-live. Unscrupulous servers
   under this scheme can manipulate the time to live but a security
   aware resolver will bound the TTL value it uses at the original
   signed value.  Separately, signatures include a time signed and an
   expiration time.  A resolver that knows the absolute time can
   determine securely whether a signature has expired.  It is not
   possible to rely solely on the signature expiration as a substitute
   for the TTL, however, since the TTL is primarily a database
   consistency mechanism and, in any case, non-security aware servers
   that depend on TTL must still be supported.

2.3.4 Special Considerations at Delegation Points

   DNS security would like to view each zone as a unit of data
   completely under the control of the zone owner and signed by the
   zone's key.  But the operational DNS views the leaf nodes in a zone,
   which are also the apex nodes of a subzone (i.e., delegation points),
   as "really" belonging to the subzone.  These nodes occur in two
   master files and may have RRs signed by both the upper and lower
   zone's keys.  A retrieval could get a mixture of these RRs and SIGs,
   especially since one server could be serving both the zone above and
   below a delegation point.

   In general, there must be a zone KEY RR for the subzone in the
   superzone and the copy signed in the superzone is controlling.  For
   all but one other RR type that should appearing in both the superzone




 
RFC 2065                DNS Security Extensions             January 1997


   and subzone, the data from the subzone is more authoritative.  To
   avoid conflicts, only the KEY RR in the superzone should be signed